1. Field of the Invention
The present invention relates to dielectric ceramics, methods for making and evaluating the same, and monolithic ceramic electronic components. In particular, the present invention relates to thin monolithic ceramic electronic components such as thin monolithic ceramic capacitors.
2. Description of the Related Art
Monolithic ceramic capacitors, as an example of monolithic ceramic electronic components relating to the present invention, are typically produced as follows.
Green ceramic sheets, each composed of a dielectric ceramic material and provided with an internal electrode pattern of a conductive material, are prepared. The dielectric ceramic material may comprise BaTiO3, for example.
A plurality of green ceramic sheets, including the above sheets provided with the internal electrode patterns, is stacked and is thermally compressed to form a green composite.
The green composite is fired to prepare a sintered composite, which has internal electrodes formed of the above-described conductive material.
External electrodes are formed on outer faces of the composite so that the external electrodes are electrically connected to predetermined internal electrodes. The external electrodes are formed, for example, by applying a conductive paste containing a conductive metal powder and a glass frit on the outer faces of the composite and baking the composite. A monolithic capacitor is thereby formed.
In order to reduce production costs of the monolithic ceramic capacitors, relatively inexpensive base metals such as nickel and copper are often used nowadays as the conductive materials for the internal electrodes. Unfortunately, the green composite must be fired in a neutral or reducing atmosphere to prevent oxidation of the base metal in the production of monolithic ceramic capacitors having such internal electrodes formed of base metals. As a result, the dielectric ceramic used in the monolithic ceramic capacitor must have resistance to reducing atmosphere.
BaTiO3-rare earth oxide-Co2O3 compositions for such dielectric ceramics having resistance to reducing atmosphere are disclosed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068. Dielectric ceramics having a high dielectric constant, a small change in dielectric constant with temperature and a long life at high-temperature load are disclosed in Japanese Unexamined Patent Application Publication Nos. 6-5460 and 9-270366.
Trends toward miniaturization and higher capacitance are noticeable in monolithic ceramic capacitors with the rapid miniaturization of electronic components as a result of recent great advances in electronics technologies.
The requirements regarding reliability for dielectric ceramics which are fired in an atmosphere which does not oxidize base metals used in internal electrodes are a high dielectric constant, a small change in dielectric constant with temperature and time, and high electrical insulation for thinner dielectric ceramic layers. The above-described known dielectric ceramics, however, do not completely satisfy these requirements.
For example, the dielectric ceramics disclosed in Japanese Unexamined Patent Application Publication Nos. 5-9066, 5-9067, and 5-9068 above satisfy the X7R characteristics in the EIA Standard and exhibit high electrical insulation, but do not always satisfy the demands of the market, namely, they may be sufficiently reliable, when the thicknesses of the dielectric ceramics are reduced to about 5 μm or less and particularly 3 μm or less.
In the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 6-5460, the particle size of the BaTiO3 powder used is large. Thus, its reliability decreases and the change in electrostatic capacitance with time increases as the thickness of the dielectric ceramic layer decreases.
Also, the reliability of the dielectric ceramic disclosed in Japanese Unexamined Patent Application Publication No. 9-270366 decreases and the change in electrostatic capacitance with time increases while applying a DC voltage as the thickness of the dielectric ceramic layer decreases.
When the same rated voltage is applied to a dielectric ceramic layer having a reduced thickness, which agrees with miniaturization and higher capacitance requirements of the monolithic ceramic capacitor, a larger electric field is applied to each layer of the dielectric ceramic. Thus, the insulating resistance at room or high temperature decreases, resulting in significantly decreased reliability. Accordingly, the rated voltage must be reduced when the thickness of the dielectric ceramic layers in the known dielectric ceramic is reduced.
There have been demands that monolithic ceramic capacitors have high insulation resistance in high electric fields and have high reliability, and that they can be used at high rated voltages even when the thicknesses of the dielectric ceramic layers are reduced.
It is known that the electrostatic capacitance of a monolithic ceramic capacitor varies over time because a DC voltage is applied in use. As the thickness of the dielectric ceramic layers decreases, the DC electric field per dielectric ceramic layer increases. As a result, the electrostatic capacitance changes more significantly over time.
Thus, there have been demands that monolithic ceramic capacitors have a small change in electrostatic capacitance when a DC voltage is applied in use.
Also, monolithic ceramic electronic components other than the monolithic ceramic capacitors have the above-described problems and demands.